1. Field of the Invention
The present invention relates to the way how the transmission rate of packets is controlled in a packet transmitting system. More particularly, it relates to an apparatus and a method in which the control parameters of the transmission rate of cells, such as an xe2x80x9cinitial cell ratexe2x80x9d (ICR), a xe2x80x9crate increase factorxe2x80x9d (RIF) and a xe2x80x9crate decrease factorxe2x80x9d (RDF), are dynamically altered in the ABR (Available Bit Rate) service of an ATM (Asynchronous Transfer Mode) network, thereby to control the transmission rate of the cells of a transmission terminal.
2. Description of the Related Art
At present, in the ATM Forum and the ITU-T (International Telecommunication Union-Telecommunication Standardization Sector), a service called xe2x80x9cABR (Available Bit Rate) servicexe2x80x9d is under study as a method in which data exchange technology based on the ATM is applied to high-speed data communication networks such as LANs (Local Area Networks) and WANs (Wide Area Networks).
In communications which utilize the ABR service, resource management cells (RM cells) are used for notifying the information items of the resources of the network to terminals. The transmission terminal sends out the RM cell each time it sends out a predetermined number of user data cells. The RM cell arrives via the network at the reception terminal, at which it is looped back to return to the transmission terminal again.
Meanwhile, an ATM switch or exchange included in the network writes the resource information items (bandwidth information, congestion information) from the ATM switch itself, into the RM cell passing therethrough, thereby to notify the information of the network side to the transmission terminal. The transmission terminal which has received the RM cell bearing the bandwidth information and the congestion information, recalculates an xe2x80x9callowed cell ratexe2x80x9d (ACR) in accordance with these information items and sends out the cells at a transmission rate not exceeding the ACR.
Besides, in establishing an ABR connection at the start of the communications, the transmission terminal reports to the network a xe2x80x9cpeak cell ratexe2x80x9d (PCR) being the maximum transmission rate, and a xe2x80x9cminimum cell ratexe2x80x9d (MCR) being the lowest transmission rate, and it negotiates about the values of these rates. Herein, the transmission terminal cannot send out the cells at a rate which is higher than the PCR determined as the result of the negotiations. On the other hand, the network guarantees the transmission terminal that transmission rate of the cells which is not lower than the MCR obtained by the negotiations. In the ABR service, accordingly, the ACR of the transmission terminal fluctuates within a range of the MCR to the PCR both inclusive (MCRxe2x89xa6ACRxe2x89xa6PCR).
Owing to the above operations, the utilization of the ABR service permits the network side to avoid congestion and to recover from congestion, and it also permits the terminal side to transmit the cells at a high transmission rate in a case where the resource of the network is idle.
At present, the operations of the transmission terminal and reception terminal which communicate by utilizing the ABR service are subjects for standardization in the ATM Forum. The main operations of the terminals will be outlined below.
FIG. 1 is a conceptual diagram of a flow control in the ABR service.
Referring to the figure, as indicated by a forward flow A, an ABR transmission terminal 1 sends out cells in a forward direction (from a transmission side to a reception side) at a rate of or below an ACR each time. On that occasion, the ABR transmission terminal 1 sends out one RM cell 4F to a switch (or ATM exchange) 3 each time it sends out a predetermined number of user data cells 2. Besides, in a case where the ABR transmission terminal 1 has received an RM cell 4B from the switch 3, it increases the value of the ACR to the amount of a predetermined constant magnitude (for example, a magnitude of RIFxc3x97PCR) by utilizing the RIF, on condition that a xe2x80x9ccongestion indicatorxe2x80x9d (CI) bit and a xe2x80x9cno increasexe2x80x9d (NI) bit, not shown, contained in the RM cell 4B are respectively set at xe2x80x9c0xe2x80x9d (not congested) and xe2x80x9c0xe2x80x9d (increase not allowed). In contrast, on condition that the CI bit is set at xe2x80x9c1xe2x80x9d (congested), the ABR transmission terminal 1 decreases the value of the ACR to the amount of a predetermined constant magnitude (for example, a magnitude of RDFxc3x97PCR) by utilizing the RDF. Further, after the above operation, the ABR transmission terminal 1 makes a comparison between the magnitudes of the value of an xe2x80x9cexplicit cell ratexe2x80x9d (ER) written in the RM cell 4B and the value of the ACR obtained by the above calculation, and it sets the value of smaller magnitude as a new ACR. On this occasion, the value of the ACR must fall within a range of MCRxe2x89xa6ACRxe2x89xa6PCR.
As indicated by a backward flow B in FIG. 1, an ABR reception terminal 5 terminates the user data cells 2 sent in from the transmission side. Also, the ABR reception terminal 5 loops back the received RM cell 4F and sends it out toward the ABR transmission terminal 1 in a backward direction (from a reception side to a transmission side). In this case, on condition that an xe2x80x9cexplicit forward congestion indicationxe2x80x9d (EFCI) bit expressive of xe2x80x9cbeing congestedxe2x80x9d is set at xe2x80x9c1xe2x80x9d in the user data cell 2A received immediately before the reception of the RM cell 4F, the ABR reception terminal 5 sets xe2x80x9c1xe2x80x9d at the CI bit contained in the RM cell 4F to-be-looped-back, and it sends out the RM cell 4 after the setting, as the RM cell 4B in the backward direction.
Next, the functions of the ATM exchange or switch 3 in the ABR service will be explained.
The operations of the network side or the ATM exchange 3 for implementing the ABR service are broadly divided into two categories, which are respectively called the xe2x80x9cEFCI modexe2x80x9d and xe2x80x9cER modexe2x80x9d.
In the EFCI mode, subject to the congestion of the network, the ATM exchange 3 sets xe2x80x9c1xe2x80x9d at an EFCI bit in a user data cell 2 which is to pass through the ATM exchange 3, and it causes this cell 2 to pass therethrough.
In the ER mode, the ATM exchange 3 calculates an ER being an explicit rate which is used for limiting the ACR on the side of the transmission terminal 1, to a specified value in accordance with the internal resources and congested situation of the network, and it writes the calculated value into a forward or backward RM cell 4 which is to pass through the ATM exchange 3. Herein, the ATM exchange 3 compares an ER value already contained in the RM cell 4, with the ER value calculated by itself, and it sets the calculated ER value in the ER field of the pertinent RM cell 4 on condition that this calculated ER value is smaller.
Further, it is possible that the ATM exchange 3 or the reception terminal 5 generates an RM cell separately from an RM cell 4F sent out of the transmission terminal 1, whereupon it sends out the generated RM cell to the side of the transmission terminal 1.
As stated before, the transmission terminal 1 increases or decreases the value of the ACR with reference to the set value of the CI bit or NI bit contained in the RM cell 4B, by utilizing the parameter RIF or RDF. In this regard, it has heretofore been the actual circumstances that the values of the parameters RIF and RDF are determined when a call is set (that is, when an ABR connection is established), and that they cannot be thereafter altered.
Accordingly, the decremental or incremental magnitude of the ACR is constant irrespective of the degree of congestion of the network for the reason that the parameters RIF and RDF are constant. In other words, the decremental magnitudes of the ACR become equal in both a case where the network is excessively congested and a case where it is lightly congested. Likewise, the incremental magnitudes of the ACR are equal without regard to the degrees of non-congestion of the network.
However, the decremental width of the ACR should preferably be made larger in the case of the excessively congested state of the network, whereas it should preferably be made smaller in the case of the lightly congested state of the network. Besides, even in the case of the non-congested state of the network, the incremental width of the ACR should preferably be changed in accordance with the degree of non-congestion.
The parameter ICR mentioned before is a rate to which the transmission terminal 1 must conform in transmitting cells at the start of the transmission, or after having failed to transmit cells at a certain predetermined time interval. The value of the parameter ICR is also determined when the call is set (that is, when the ABR connection is established). In this regard, the transmission terminal 1 lowers the ACR to the ICR on conditions that an inequality ACR greater than ICR holds before a forward RM cell 4F is sent out, and that an elapsed time since a forward RM cell 4F was sent out last is greater than a predetermined allowable time period. As a parameter expressive of the predetermined allowable time period, an ADTF (ACR Decrease Time Factor) is employed in the ABR service.
Accordingly, in a case where the transmission terminal 1 has failed to send out the forward RM cell 4F in spite of the lapse of the time period ADTF since the last sending-out of the forward RM cell 4F, it cannot but set the value of the ACR at the above ICR of the constant value without regard to the congested state of the network in the case where it was sending out cells. For the transmission terminal 1, however, it is desirable that the network side changes the value of the ICR in accordance with the congested state of the network in the case where this terminal 1 was transmitting the cells. In other words, in the case where the transmission terminal 1 has failed to send out the next forward RM cell 4F in spite of the lapse of the time period ADTF, it should desirably restart its transmission at the ACR of larger value subject to the non-congested state of the network, and conversely, it should desirably restart its transmission at the ACR of smaller value subject to the congested state of the network.
In view of the problems stated above, the present invention has for its object to provide an apparatus and a method for a transmission rate control in which the transmission rate control parameters of an ABR service, such as the ICR, RIF and RDF, are dynamically changed in accordance with the situations of uses of network resources, whereby the ACR of a transmission terminal is controlled so as to be set at a value reflecting the situation of congestion of a network.
A transmission rate control apparatus in the first aspect of the present invention comprises recalculation means for recalculating a value of a second transmission-rate control parameter on the basis of resource information of a network by using a value of a first transmission-rate control parameter which is set in an inputted RM cell; comparison means for comparing the value of the second transmission-rate control parameter obtained by the recalculation means, with the value of the first transmission-rate control parameter; and parameter resetting means for rewriting the value of the first transmission-rate control parameter set in the input RM cell, into the value of the second transmission-rate control parameter, only when the value of the second transmission-rate control parameter has been decided smaller than the value of the first transmission-rate control parameter by the comparison means.
In operation, the recalculation means recalculates the value of the second transmission-rate control parameter on the basis of the resource information of the network by using the value of the first transmission-rate control parameter which is set in the inputted RM cell. The comparison means compares the value of the second transmission-rate control parameter obtained by the recalculation means, with the value of the first transmission-rate control parameter. The parameter resetting means rewrites the value of the first transmission-rate control parameter set in the input RM cell, into the value of the second transmission-rate control parameter, only when the value of the second transmission-rate control parameter has been decided smaller than the value of the first transmission-rate control parameter by the comparison means.
A transmission rate control apparatus in the second aspect of the present invention comprises recalculation means for recalculating a value of a second transmission-rate control parameter on the basis of resource information of a network by using a value of a first transmission-rate control parameter which is set in an inputted RM cell; comparison means for comparing the value of the second transmission-rate control parameter obtained by the recalculation means, with the value of the first transmission-rate control parameter; and parameter resetting means for rewriting the value of the first transmission-rate control parameter set in the input RM cell, into the value of the second transmission-rate control parameter, only when the value of the second transmission-rate control parameter has been decided larger than the value of the first transmission-rate control parameter by the comparison means.
In operation, the recalculation means recalculates the value of the second transmission-rate control parameter on the basis of the resource information of the network by using the value of the first transmission-rate control parameter which is set in the inputted RM cell. The comparison means compares the value of the second transmission-rate control parameter obtained by the recalculation means, with the value of the first transmission-rate control parameter. The parameter resetting means rewrites the value of the first transmission-rate control parameter set in the input RM cell, into the value of the second transmission-rate control parameter, only when the value of the second transmission-rate control parameter has been decided larger than the value of the first transmission-rate control parameter by the comparison means.
According to the present invention, control parameters for the transmission rate of the ABR, such as the ICR, RIF and RDF, concerning the control of the transmission rate of the cells of the transmission terminal in the ABR service are dynamically changed in accordance with the situations of the resources of the network. It is therefore permitted to appropriately control the transmission rate of the cells of the transmission terminal in accordance with the situations of uses of the network resources, such as the situation of congestion of the network.